code: def test_optical_density_zeromean():
      """Test that optical density can process zero mean data."""
      raw = read_raw_nirx(fname_nirx, preload=True)
      raw._data[4] -= np.mean(raw._data[4])
      raw._data[4, -1] = 0
      with np.errstate(invalid="raise", divide="raise"):
          with pytest.warns(RuntimeWarning, match="Negative"):
              raw = optical_density(raw)
      assert "fnirs_od" in raw
  
file_path: mne-python/mne/preprocessing/nirs/tests/test_optical_density.py
imports: import numpy
  import pytest
  from numpy.testing import assert_allclose
  from mne.datasets import testing
  from mne.datasets.testing import data_path
  from mne.io import BaseRaw, read_raw_nirx
  from mne.preprocessing.nirs import optical_density
  from mne.utils import _validate_type
label_desc: Use image node_id 2 for calling a global function with example usage: test_optical_density_zeromean() without return types